Material & Methods

SEATRACK utilizes light-level geolocator data loggers (historically known as Global Location System, Global Location Service and Global Location Sensing as well as GLS logger) to investigate the year-round movement of seabirds. These small, archival, light-weight and low-cost, but low accuracy global tracking devices (Wilson et al. 1992) are easily attached to a birds leg-band and can collect data for several years at the time (depending on logger model).

Figure 1: Some of the GLS logger models being used by project participants, mounted on coded leg-bands. Photo: Hálfdán Helgason

A GLS logger is typically made up of five components; a light sensor, an internal clock, a memory chip, a battery, and connection pins, all encapsulated in an epoxy shell. The sensor measures light intensity every minute and stores the maximum light level every 5 or 10 minutes. The connection pins, primarily used for downloading data and programming sampling regimes, can also register whether the logger is submerged in salt water or not, thus providing information concerning the bird’s activity. Some logger models also measure water temperature when submerged. Logger models vary with respect to weight (1-2.5 g), size (height 15-20 mm, width 9-14 mm) and battery capacity (1-5 years). The smallest models are most suitable for the smaller species (such as the little auk) but have more limited battery capacity. The low weight of the loggers (<1 % of the birds’ body weight) combined with specially adapted mounting rings minimise any impact on the birds behaviour and condition, e.g. the bird can still withdraw its feet into its plumage and extra flight costs are expected to be minimal (Figure 1). Swimming and diving may, however, be potentially affected. However, there are currently no indications of such impacts on seabirds, despite these methods having been tried and tested for several years on small diving species (Brlík et al. 2019; Geen, Robinson, and Baillie 2019; Nicoll et al. 2022).

A main drawback of GLS technology is that each bird must be recaptured to download the data. Consequently, only the adult breeding part of the population can be tracked with this technology. However, SEATRACK in its second phase (2019-2022) started to deploy GLS also on fledging chicks in some colonies in order to collect information on immature individuals when these chicks are eventually recaptured as breeding adults back in the colony. But, the need for recapture precludes information collection on failed individuals (As these will never be recaptured). Also, spatial location accuracy and precision using GLS (~ 100 s of kms) is far lower than using ARGOS or GPS tracking (Phillips et al. 2004; Merkel et al. 2016; Halpin et al. 2021) due to the inherent nature of location estimation using ambient light (Lisovski et al. 2012). Furthermore, location accuracy or availability is further reduced during certain periods of the year, e.g. during the equinox periods and during periods of continuous light (midnight sun) or darkness (polar night).

Eight different datasets (b-h) have been produced, in part for this report, using GLS logger data. The steps to derive at these datasets and their rational are detailed in the following sections and summarized in Figure 2 . All processing and analyses described in the following sections were conducted using R Statistical Software v.3.6.3 - 4.2.2 (Computer Program 2022).

Figure 2: Overview of methodological workflow in eight steps, resulting in the eight data products utilized throughout this report, their references, and the number of species for which they have been calculated.